This commit allows parseRoutingInfo to return an error when parsing a
routing info field whose length is not a multiple of 51 bytes, rather
than crash.
This commit refactors parsing each of the tagged fields of an invoice
into their own method. This makes the code easier to read and will allow
us to introduce unit tests for each parsing method.
This commit renames the invoice field Expiry to expiry, and changes
the type from time.Time to time.Duration. Getting the value of the
field will now have to be done using the getter Expiry(), which
will also return the default invoice expiry (3600s) if it is not set
explicitly by the the invoice.
